Phase change materials (PCM) are capable of storing heat energy in the form of latent heat. Such materials undergo a phase transition when heat is supplied or removed, e.g., a transition from the solid to the liquid phase (melting) or from the liquid to the solid phase (solidification) or a transition between a low-temperature and high-temperature modification or a hydrated and a de-hydrated modification or between different liquid modifications. If heat is supplied to or removed from a phase change material, on reaching the phase transition point, the temperature remains constant until the material is completely transformed. The heat supplied or released during the phase transition, which causes no temperature change in the material, is known as latent heat.
Unfortunately, the heat conductivity of most phase change materials is rather low. As a consequence, the charging and discharging of a latent heat storage device is a relatively slow process. This problem can be overcome by providing a latent heat storage composite wherein the phase change material is combined with an auxiliary component with high thermal conductivity, e.g., graphite.
Granted European Patent No. EP 0914399 B1 discloses a composite for storage of latent heat and the process of manufacturing such a composite. The composite consists of an inert graphite matrix with a bulk density of more than 75 grams per liter (g/l) which has been infiltrated under vacuum with a solid/liquid phase change material (PCM). The graphite matrix has a high porosity and allows a high PCM loading of up to at most 90% by volume without being destroyed by the change in volume occurring during the phase transition. A high PCM loading in the composite is needed to achieve a high energy density.
One advantage of this composite is the use of graphite as matrix material, which by its nature has a high thermal conductivity and, since it is substantially chemically inert, imposes scarcely any restrictions on the choice of the PCM.
The graphite matrix is made by compressing expanded graphite material to a density between 75 and 1500 g/l, preferably between 75 and 300 g/l. The storage composite is obtained by vacuum infiltration of the PCM into the preformed matrix. The matrix is made by compressing expanded graphite material with a bulk density of two grams per liter into the shape of a cylinder with a diameter of 42 mm and a height of 10 mm. Prior to the infiltration, the matrix is evacuated to a pressure of 10 mbar or below, and the PCM is heated to a temperature which is preferably between 10 and 40 Kelvin (K) above the melting point, but at most up to the evaporation temperature of the PCM. As a result of a valve leading to the PCM vessel being opened, the molten PCM, which is present in excess, is sucked into the graphite matrix. Then, the storage composite is preferably cooled to below room temperature, in order to reduce the escape of PCM gases until the storage container is closed.
An alternative process for vacuum infiltration of a matrix made by compression of expanded graphite material was disclosed in published U.S. Publication No. 2002-060063. The process comprises the steps of partially or completely immersing the matrix, which is fixed inside an infiltration vessel, under atmospheric pressure in a molten phase change material, and evacuation of the infiltration vessel until the desired degree of loading of the matrix with the PCM has been achieved.
The vacuum infiltration process can be continued until the residual porosity of the composite is approximately 5% by volume. This residual porosity can be reached after an infiltration period of up to approximately five days, preferably of approximately up to four days. The graphite matrix expediently has a density of about 75 to about 1500 g/l, preferably about 75 to about 300 g/l, particularly preferably approximately of about 200 g/l.
There are several phase change materials, especially hydrophilic ones, which do not readily infiltrate into a porous matrix formed by compression of expanded graphite material because of a rather high interfacial energy between the phase change material and the graphite material of the matrix. When the interfacial energy between both PCM and graphite matrix is high, the matrix is not or only scarcely wetted by the PCM.
Due to a phenomenon known as negative capillary effect, this problem tends to be more severe the smaller the radii of the pores are. The low PCM loading of a matrix with small pores results in a low energy density of the storage composite, and therefore a low efficiency.
Another drawback of the prior art is the need to carry out the infiltration under vacuum which makes the equipment for the infiltration rather complicated and expensive.
Another type of latent heat storage composite is disclosed in U.S. Publication No. 2005-258394. Within this composite, flakes of natural graphite or/and synthetic graphite having a high anisotropy of thermal conductivity and a high aspect ratio form the auxiliary heat conducting component.
Such a composite is obtained, for example, by infiltration of the liquid phase change material into a bed containing graphite flakes as a bulk good. Infiltration can be supported by vacuum or pressure, but this is not necessary.
In contrast, preparation of an analogous composite by infiltration of the phase change material into a bed containing expanded graphite material as bulk good is rarely possible. Due to the high interface energy between the expanded graphite material and the PCM, it is extremely difficult to infiltrate expanded graphite material as bulk good with a liquid PCM without foaming and floating of the expanded particles. Therefore, there is a need to improve the wettability of the expanded graphite particles by the PCM to facilitate infiltration.